The present invention relates generally to overcurrent and overvoltage indication for electrical devices. More particularly, the present invention relates to fuses having opened-fuse indication.
Known fuses exist that open if a short circuit occurs or if a current overload condition occurs. For example, some known fuses include a short circuit element in series with a current overload or time delay element. The combination of fuse elements electrically communicates with a pair of terminals, which respectively electrically communicate with a pair of electrically conductive end caps. The end caps of the fuse typically snap-fit into well-known fuse clips.
When one of the above described types of fuse elements opens, it is desirable that the fuse indicates its opened state to an operator. For this reason, known fuses also exist that provide opened-fuse indication. There are various types of opened-fuse indicators. Some known fuses provide a spring loaded mechanism, wherein the opened-fuse triggers a spring that moves a plunger to a more visible location. Other fuse indicators provide a resistive circuit in parallel with the fuse element, wherein the highly resistive circuit includes a light emitting diode (“LED”) or lamp in parallel with the fuse element. Normally, virtually all the current flows through the fuse element such that the little amount of current that does travel through the highly resistive circuit does not illuminate the LED. When the fuse opens, the current is forced through the highly resistive circuit, illuminates the LED and provides opened-fuse indication.
Littelfuse, Inc., the assignee of the present invention, provides an opened-fuse indicator which has a clear or transparent plastic lens that makes an internal fluorescein coated indicator coil visible to an operator. The opened-fuse indicator provides a circuit that is in parallel with a fuse element. The resistance of the indicator coil is substantially higher than the resistance through the fuse element, so that current normally travels through the fuse element. When the fuse element melts, the main circuit opens, and current shunts through the resistive indicator coil, causing the coil to heat up and vaporize the fluorescein into a colored gas. The colored gas collects on the interior of the transparent plastic lens and provides opened-fuse indication.
Another type of opened-fuse indicator includes a clear or transparent plastic lens that makes an internal ball of white “gun cotton” visible. Gun cotton ignites and disappears when subjected to flames, sparks or temperatures of about 280° F. (138° C.). An igniter wire, which is in parallel with a fuse element, runs through the gun cotton. A black background exists behind the gun cotton, which is normally not visible to the operator. When the fuse element melts, current shunts through the igniter wire, and the wire heats to a temperature above the ignition temperature of the gun cotton. The gun cotton burns away, exposing the black background and providing opened-fuse indication.
A further type of opened-fuse indicator includes a flexible label attached to the exterior of the fuse body. The label has a semi-conductive layer fixed to the outside of the fuse body, which is connected in parallel with a fuse element. A temperature responsive layer is disposed on the outside of the semi-conductive layer and normally blocks the operator from seeing the background. The resistance in the semi-conductive layer is substantially higher than that of the fuse element.
In normal operation, most of the current runs through the fuse element, and the small amount of current that runs through the semi-conductive layer does not produce enough heat to raise the temperature of the responsive layer above its transition temperature. If the current in the circuit exceeds the amperage permitted by the fuse element, the fuse opens and allows current to shunt though the semi-conductive layer. The semi-conductive layer heats the temperature responsive layer above its transition temperature, whereby the responsive layer changes to a generally transparent state and permits a color of the semi-conductive layer to become visible.
While the above described indicating fuses have provided adequate opened-fuse indication, a continuing need exists to provide more dependable and cost effective indicating fuses. Also, a continuing need exists to provide an opened-fuse indicator that indicates at a lower operating temperature.
Most of the currently available opened-fuse indicators are provided inside of the electrical fuse. Consequently, size becomes an issue. Typically, the lower the rating of the fuse, the smaller the size of the fuse. It becomes increasingly difficult to place the above-described fuse indicators inside of the fuses for increasingly smaller fuses. A need therefore exists to provide an opened-fuse indicator for lower rated fuses.
Further, a need exists to provide overcurrent or overvoltage indication for electrical devices besides fuses. That is, it is desirable to know if particular electrical devices have been subjected to a surge current or voltage.